Aqueous solutions of hydrogen iodide and iodide salts and compounds have found broad use in pharmaceuticals, disinfectants, bactericides and fungicides. Hydriodic acid, the azeotrope which contains 56.9% HI, is usually preferred over HI itself. HI is a strong reducing agent that is easily oxidized to iodine on exposure to light and air. A colorless solution when freshly made rapidly turns yellowish or brown and sometimes becomes opaque from oxidation. The oxidized solution may be regenerated by adding a reducing agent such as hypophosphorous acid. However, the resulting phosphorous acid remains as an impurity unless the solution of HI is distilled.
Aqueous solutions of iodides have been made by reacting iodine with hydrogen to form hydrogen iodide or hydriodic acid and then reacting the hydrogen iodide or hydriodic acid with cations or other reactive materials to form salts and other compounds containing iodine. Iodine can be reacted with hydrogen over a platinum catalyst or with wetted phosphorus, hydrazine and other reducing agents to convert the iodine to iodides.
Electrodialysis has also been used to prepare iodides. This technology comprises the transport of ions through ion permeable membranes as a result of an electrical driving force. The process is usually carried out in an electrochemical cell having at least an anolyte compartment containing an anode and an anolyte separated by an ion permeable membrane from a catholyte compartment containing a cathode and a catholyte. The ion permeable membrane can be permeable to cations or anions. The anion permeable membranes usually have fixed positive charges and, as the name implies, are permeable to anions and relatively impermeable to cations. The cation permeable membranes usually have fixed negative charges and are permeable to cations. Bipolar membranes are composites of anion and cation permeable membranes, one surface of the bipolar membrane being anionic and the other surface cationic. The bipolar membranes are frequently used to split water to provide hydrogen ions and hydroxyl ions to effect ionic neutralization in a multicompartmented electrochemical cell. The bipolar membranes are normally used between an anion permeable membrane and a cation permeable membrane. Oxidation reactions occur at the cell anode and reduction reactions occur at the cell cathode. Halides are converted to halogens at the cell anode and halogens are converted to halides at the cell cathode. There is always some reverse migration through ion permeable membranes. It is essential in an electrodialytic process that the membranes are selectively permeable, not readily fouled and the membranes separating compartments are arranged whereby the flow of ions provides for ionic neutralization. The electrochemical cells can be separated by all cation permeable membranes, all anion permeable membranes or various combinations of cation, anion, bipolar membranes and porous diaphragms.
A specific method for the electrolytic production of hydrogen iodide is disclosed in U.S. Pat. No. 4,053,376 using an electrolytic cell having an anolyte and catholyte separated by a cation permeable membrane. Iodine is added to the catholyte and hydrogen ions are electrotransported from the acidic anolyte to the catholyte. Attempts to operate this electrolytic process over several hours are not always successful because crystals containing iodine tend to form on the anode side of the membrane and also in the membrane. This results in reducing the permselectivity of the membrane and in forming iodine in the anolyte. The iodine formed, being much more soluble in a solution of an iodide than in water, tends to dissolve in the catholyte.
In the electrochemical process, the number of molecules or ions in solution remains unaltered since it is believed that the molecules of iodine added to the catholyte converts I ions into I.sub.3 ions. The exact constitution of these polyiodides is not very clear. For example, when a solution of iodine in potassium iodide is evaporated, large black crystals are obtained having a formula approximating KI.sub.3. The iodine molecule may be added on to the iodine ion thus [II.sub.2 ]. In concentrated solutions of potassium iodide, large quantities of iodine (up to eight or more atoms per molecule) can be dissolved. When potassium iodide is added to a solution of a cupric salt, cuprous iodide is at once precipitated and forms a polyiodide with the liberated iodine. When an iodide is exposed to light or oxygen, free iodine is formed which apparently adds to the iodide ion to form a polyiodide.
The foregoing characteristics of iodine and iodides essentially preclude producing the hydriodic acid, alkali iodides and other iodide compounds substantially free of iodine. It is a primary object of the instant invention to alter this siutuation, i.e. to provide a process for making iodides substantially free of iodine.
A further object of the instant invention is to provide a method for electrodialytically producing an iodide solution that is colorless, substantially iodine-free, with no contamination of the solution with a reducing agent or its oxidation products.